ADM_formalism

ADM formalism

The formalism supposes that spacetime is foliated into a family of spacelike surfaces Sigma_t, labeled by their time coordinate t, and with coordinates on each slice given by x^i. The dynamic variables of this theory are taken to be the metric of three dimensional spatial slices gamma_{ij}(t,x^k) and their conjugate momentapi^{ij}(t,x^k). Using these variables it is possible to define a Hamiltonian, and thereby write the equations of motion for general relativity in the form of Hamilton's equations.

In addition to the twelve variables gamma_{ij} and pi^{ij}, there are four Lagrange multipliers: the lapse function, N, and components of shift vector field, N_i. These describe how each of the "leaves" Sigma_t of the foliation of spacetime are welded together. These variables are nondynamical, and their "equations of motion" are constraint equations that enforce invariance under time reparameterizations and coordinate changes on the spatial slices, respectively.